The invention relates to an electroluminescent device comprising a layer of an electroluminescent material with an active layer of an organic material, which layer is situated between a first and a second pattern of electrodes, at least one of the two patterns being transparent to light to be emitted by the active layer, and a first pattern comprising a material which can suitably be used to inject charge carriers by applying a bias voltage or bias current for emitting, and said device comprising a control unit for applying the bias voltage across a part of the active layer or for setting the bias current through a part of the active layer.
The invention further relates to a display device comprising such an electroluminescent device.
Electroluminescent (EL) devices may be used as illumination sources and, for example, in displays and indicator lamps. For the active layer in such structures use is increasingly being made of organic materials, such as semiconducting organic polymers. This increases the number of possible materials for use in this type of devices.
The active layer and the two electrode layers may jointly constitute one light-emitting diode (LED), but the electroluminescent device preferably comprises a number of LEDs, for example in the form of segmented light-emitting surfaces or light-emitting surfaces arranged in the form of a matrix, as intended for a display device as described, for example, in WO 96/36959 (PHN 15.320), or combinations thereof.
The operation is based on the recombinations of electron-hole pairs which are injected in the semiconductor material (during operation in the forward direction) from electrodes located on either side of the active layer. Due to these recombinations, energy in the form of (visible) light is released, which is a phenomenon referred to as electroluminescence. The wavelength and hence the color of the emitted light is also determined by the band gap of the (semiconductor) material.
Dependent upon the way in which the electroluminescent device is used, problems occur typically when this type of display devices is used in varying ambient light conditions.
For example, when such a display device is used in "automotive" display devices (motor vehicles), it must not dazzle in the dark, but, on the other hand, its light output in daylight must be sufficient to ensure a satisfactory degree of readability. Such display devices are generally of the so-called negative type (combination of a transmissive LCD and an LED-backlight, direct emitting electroluminescent display devices). Application in so-called positive-type display devices (a combination of a transflective LCD and an LED-backlight), such as widely used in portable telephones, requires, however, an additional light source in the dark to ensure a sufficient degree of readability.